A long standing and pervasive assumption held by lay persons and psychologists alike is that an individual's behavior is generally predictable across a variety of situations and over time. A number of longitudinal studies have been conducted over the past 50 years to test the developmental hypothesis concerning the stability of personality. The purpose of this project is to review and integrate the findings from the various longitudinal studies concerning the evidence for stability and change in personality over the life span. Attention is given to the assumptions associated with assessing stability and change and to the methodological and psycho-social conditions that influence whether stability or change is observed.